The present invention relates to an insulator, particularly for electric transmission and distribution lines, with improved resistance to flexural stresses.
More particularly, the invention relates to a hollow insulator for pole heads for supporting pylons in electrical transmission and/or distribution lines.
It is known that hollow insulators are generally constituted by a tube made of composite material, typically fiberglass-reinforced plastic, around which insulating ribs made of silicone material are arranged; suitable metallic flanges are also fixed to one or both ends of the tube and allow to couple the insulator to an additional element, such as for example another insulator or the frame of a supporting pylon.
In the current state of the art, the flanges are fixed to the ends of the insulator in most cases by gluing: in particular, depending on the requirements and/or on the specific applications and on the geometric configuration of the flanges, gluing can be performed on the internal surface of the insulator, on the external surface, or on both.
This solution entails several drawbacks, the critical importance of which becomes apparent when the insulator is installed and is required to withstand certain structural loads.
In particular, in the field of electrical transmission lines, new applications require the entire pole head of the supporting pylons to be formed with hollow insulators of the above-described type, so as to entrust to a single component both the structural function and the dielectric function; the purpose of this solution is to drastically reduce the lateral dimensions of the pylon, thus reducing its environmental impact.
In these applications, the insulators are required to perform particular demanding structural tasks; in particular, the type of stress whereto they are subjected is mainly flexural and becomes critically apparent at the end flanges, where the applied flexural moment is highest.
The insulated tube made of composite material is in fact inherently able to withstand much higher flexural moments in sections not close to the flange than proximate to the coupling between the tube and the flange: the flexural moments applied to the tube, however, are discharged from the tube onto the interface for connecting the flange and onto the flange itself, determining regions of high stress concentration especially at particular regions of the interface between the flange and the tube. Specifically, the transfer of flexural stresses from the composite tube to the metal flange is entrusted to the gluing, and although the tube is capable of withstanding the flexural moment applied to it, the weakest element of the coupling between the tube and the flange, i.e, the gluing element, is unable to withstand said flexural moment. Accordingly, the stresses tend to cause, even for low values of the flexural moment applied to the tube, not only cracks in the layer of adhesive but even a mutual separation of the tube and the flange, causing a consequent problem.
In particular, experimental tests and numerical analyses show that the loadbearing ability of the coupling is insufficient with respect to the maximum flexural moments generated by the loads that affect the pole head.
One of the solutions that can be adopted in order to obviate this drawback is the use of composite tubes whose diameters and thicknesses are considerably oversized; this solution is clearly disadvantageous both from the point of view of costs, which would increase considerably, and from the point of view of visual impact, which would be negatively affected by the increased dimensions of the insulator.
Another solution for improving the flexural performance of conventional insulators might be achieved, at least theoretically, by appropriately enlarging the portions of tube that are in contact with the flanges. However, this solution entails a certain complication of the process for winding the fibers of which the tubes are made (so-called filament winding), consequently entailing a considerable increase in production costs and therefore ultimately producing a product which is not competitive from the economical point of view.
It should be observed that in any case the solution that includes gluing has the severe drawback that it has a brittle fracture mode, i.e., structural failure becomes apparent suddenly and without warning and practically coincides with exit from the elastic range. For applications such as the insulating pole head it is instead desirable for failure to occur "plastically", i.e., for the structure, after leaving its elastic range, to be capable of maintaining extreme loads by undergoing even great permanent deformations but with the undisputed advantage, in terms of safety, of maintaining integrity.